Access denied on: Napa tool/ SharePoint 2013 apps

Recently I got the "Access Denied" issue when I try to access the lists data from the Visual Studio 2012 apps and Napa tool.

To get the access permissions from the Napa tool:
You will get the below popup up window once you clicked on "properties" tab[see below fig] and here you can give the access according to your functionality.

To get the access permissions from the visual studio 2012 apps. 
you will get the below thing once double clicked on "AppManifest.xml" file and then set up the permissions according to your functionality like set the "Read" Permissions to the web if you want to only read permissions etc..

Please refer the msdn article to get an idea on permissions:

http://msdn.microsoft.com/en-us/library/fp142383(v=office.15).aspx